As dimensions for microelectronics are reduced and the demand for such materials increase, thin semiconductor films having uniform composition and uniform thickness over a larger substrate area are increasingly desirable. A common process used for depositing thin films on a substrate is Chemical Vapor Deposition (CVD), which provides for deposition of relatively uniform films over complex device topography. In a typical CVD process, the substrate is exposed to two or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired thin film.
Despite improvements of CVD over previous deposition techniques, CVD has several disadvantages. For example, because CVD is flux-dependent, deposition conditions such as substrate temperature, pressure, and gas flow rate must be accurately and consistently maintained to produce a desired thin film of uniform thickness. Additionally, CVD tends to incorporate undesired reaction products into the deposited thin film, thereby diminishing the purity of the thin film.
Atomic Layer Deposition (ALD), which represents a variant of CVD, is a contemporary technology for depositing thin films now emerging as a potentially superior method of achieving highly uniform, conformal film deposition. ALD is a process wherein conventional CVD processes are divided into separate deposition steps to construct the thin film by sequentially depositing single atomic monolayers in each deposition step. The technique of ALD is based on the principle of the formation of a saturated monolayer of reactive precursor molecules by chemisorption. A typical ALD process consists of injecting a first precursor for a period of time until a saturated monolayer is formed on the substrate. Then, the first precursor is purged from the chamber using an inert gas. This is followed by injecting a second precursor into the chamber, also for a period of time, thus forming a layer on the wafer from the reaction of the second precursor with the first precursor. Then, the second precursor is purged from the chamber. This process of introducing the first precursor, purging the process chamber, introducing the second precursor, and purging the process chamber is repeated a number of times to achieve a film of a desired thickness.
ALD thin films may be deposited using single-wafer reactors with the reactive gas precursors injected into the process chamber. However, a major disadvantage of a single-wafer reactor is that it has a significantly diminished commercial value because of its relatively low throughput. Another disadvantage is that the process chamber must be purged before each individual gas precursor is introduced.